Ethylbenzene is used primarily for the production of styrene monomer obtained through dehydrogenation. Currently, much of the ethylbenzene produced is obtained by alkylation of benzene with ethylene under a variety of alkylation conditions. One type of alkylation process involves vapor phase reactions in which benzene is alkylated with an olefin such as ethylene under high temperatures and pressures over an aluminosilicate catalyst in a multiple bed reactor.
A significant problem in the production of ethylbenzene by alkylation of benzene with ethylene under high temperatures and pressures is the production of undesired byproducts, including polyalkylbenzenes such as polyethylbenzenes (PEB's) and multi-ring compounds known collectively as "heavy residue". Although the production of PEB's and heavy residue may be limited to some extent by supplying a stoichiometric excess of benzene to the reactor, significant quantities of these materials are nevertheless generated during the alkylation process.
In conventional vapor phase alkylation of benzene with ethylene, the reactor effluent is subjected to successive fractionations to separate benzene, ethylbenzene, PEB's and heavy residue to separate streams. The PEB stream may be directed to a separate transalkylation reactor or recycled to the alkylation reactor.
In the case where the PEB stream is recycled to the alkylation reactor, the recycled PEB stream can constitute up to ten percent of the total feed to the alkylation reactor. Consequently, if the transalkylation activity of the catalyst is not sufficient to control the concentration of PEB's in the alkylation reactor effluent, the volume of recycled PEB's will increase and eventually become too large to manage within process capacity. Therefore, it is important that the catalyst used in the alkylation of benzene with ethylene in a process with PEB recycle to the alkylation reactor have sufficient transalkylation activity to convert recycled polyalkylbenzenes at a rate sufficient to maintain the volume of the recycled polyalkylbenzenes stream at a level within process capacity.
A bottom stream of heavy residue is withdrawn from the reactor and usually burned as fuel. The residue is typically comprised of multi-ring compounds such as naphthalenes and diphenylethanes and has little commercial value. If not removed, this heavy residue will build up in the reactor stream, eventually overwhelming the ethylbenzene production process. This residue represents a loss of raw materials, since it is useful only as fuel, decreasing the profitability of the alkylation process. This purge stream has heretofore represented about one weight percent of the output of the alkylation process based on the weight of ethylbenzene produced.
In connection with the foregoing, it has been discovered that the use of a silicalite alkylation catalyst having a crystallite size within a specified range results in increased transalkylation activity, allowing for greater control of the volume of recycled alkylbenzenes in the process. In addition, an alkylation process has been discovered that minimizes the amount of heavy residue produced in alkylation/transalkylation of aromatic hydrocarbons.